A. Field of the Invention
The present invention relates to printing blankets of the type used in printing and offset lithography, and more particularly to a novel anisotropic endless printing element having a spirally-integrated reinforced compressible tubular structure, and to a method for making the same.
B. Description of Related Art
The printing roll of Ross (U.S. Pat. No. 3,467,009) provided volume compressibility, i.e. an ability to compress in thickness without substantial increases in lateral dimensions. The roll was made by saturating an elastomer into a felted web composed of short fibers of paper or cotton linters.
In contrast to printing rolls, printing "blankets" were first so-called because they employed sheet layers in the manner of a blanket. Blanket ends were clamped into a longitudinal cylinder gap and held tightly in position over a carcass layer or sublayer. For example, the printing blanket of Duckett et al. (U.S. Pat. No. 4,093,764) employed alternating layers of short compressed fibers with elastomer. The printing blankets of Rodriguez (U.S. Pat. No. 4,303,721) and O'Rell et al. (U.S. Pat. No. 4,812,357) used separate foamed layers and stabilizing hard elastomer layers to enhance web feed characteristics and dynamic stability.
Circumferentially seamless or "endless" printing blankets have been developed in conjunction with gapless cylinders. Endless blankets are believed by the present inventors to provide advantages over prior an blankets used on gapped cylinders because they allow printing over the entire outer surface and help to minimize vibration at high rotational speeds. However, their multi-layered construction requires many manufacturing steps and close tolerances. For example, the blanket of Gaffney et al. (Can. Pat. App. 2,026,954) used separate foam, hard rubber, and optional fabric layers. The blanket of Bresson (U.S. Pat. No. 5,205,213) employed a stabilizing hard elastomer between the printing and foam layers. The blanket of Vrotacoe et al. (EP No. 92810364.7) disclosed a filament wound, elastomeric seamless blanket having a number of layers. The trend therefore appears very much to be towards having concentric, separated, layered, complex structures.